Reaction of fluoropolymer melts

ABSTRACT

Chemical reactions are carried out with molten fluoropolymer, most conveniently in an extruder, wherein the reaction zone in the extruder is isolated from the melting zone, and the molten fluoropolymer is subdivided in the reaction zone sufficiently that contact and reaction between the reactant and the molten fluoropolymer in the reaction zone is essentially free of mass transfer limitation, followed by devolatilization in isolation from the reaction zone, and cooling the devolatilized fluoropolymer.

FIELD OF THE INVENTION

[0001] This invention relates to chemical reactions involving melts ofmelt-processible fluoropolymers, e.g. to reduce unstable polymerendgroups.

BACKGROUND OF THE INVENTION

[0002] Directly after polymerization fluoropolymers have among theirendgroups certain types that are designated as unstable. Unstableendgroups may decompose or otherwise react at the temperatures at whichthe fluoropolymers are melt-processed, that is, they are thermallyunstable. They can cause bubbles or voids in the processedfluoropolymer. They may also be a source of hydrogen fluoride in thefinished articles of melt-processing, for example by exposure of thearticles to atmospheric moisture. That is, they are hydrolyticallyunstable. Among these unstable endgroups are: —COOH, which candecarboxylate at processing temperatures, generating carbon dioxide, apotential source of bubbles and voids; —COF, which though more thermallystable, hydrolyzes easily to —COOH and HF; —CF═CF₂, which oxidizes to—COF; and —CONH₂, which although desirable in certain applications dueto its greater thermal stability than —COOH ends and greater hydrolyticstability than —COF ends can react or decompose into —COOH, —COF, or—CF═CF₂ groups, often with color formation.

[0003] Fluorination is among the ways to reduce the number of unstableendgroups in fluoropolymers that are disclosed. Such fluorinationconverts the non-perfluorinated unstable end groups to the highly stableperfluorinated end group —CF₃. British Patent 1,210,794 describescontacting solid fluoropolymer with fluorine. Because of the solid andgas phase nature of this system, particle size and efficiency of contactbetween the gas and the solid will affect the time needed to achieveunstable endgroup reduction. Contact times on the order of hours aretypical of fluorination of solid polymer.

[0004] U.S. Pat. No. 4,626,587 discloses that as-polymerizedmelt-processible fluoropolymer such as tetrafluoroethylene(TFE)/hexafluoropropylene (HFP) copolymer (FEP), has an additionalsource of instability, namely relatively unstable linkages in thepolymer backbone arising from adjacent HFP units in the polymerbackbone. This patent teaches removal of this instability by subjectingthe fluoropolymer to high shear in a twin-screw extruder, the twinscrews preferably containing kneading blocks. The unstable end groups ofthe fluoropolymer and/or any poor (dark) color present in thefluoropolymer after this high shear treatment are eliminated by anafter-treatment of the fluoropolymer, such as fluorination as describedin British Patent 1,210,794 or humid heat treatment such as disclosed inU.S. Pat. No. 3,085,083, wherein water (steam) converts —CF₂COOH endgroups to more stable —CF₂H end groups. Fluorination of the extrudedcubes of fluoropolymer changes the dark as-extruded color of the cubesto white. U.S. '587 also discloses the addition of 1 wt % water to thetwin-screw extruder to hydrolyze acid fluoride end groups (paragraphbridging cols. 7 and 8), this amount of water corresponding to a molarexcess of 751× with respect to the 37 acid fluoride groups present/10⁶carbon atoms when the water was not added.

[0005] European Patent Application 0 928 673 A1 (WO98/09784) disclosesthe removal of unstable end groups, unstable bonds in polymer backbonesof fluoropolymers, and poor color from the fluoropolymer by subjectingthe fluoropolymer to a melt kneading step. This publication observesthat the use of a twin-screw extruder in U.S. '587 provides too short aresidence time and requires the after-treatment to remove unstable endgroups and poor color and overcomes this shortcoming by using a kneader,which has a higher usable volume ratio (usable space/actual space) thana twin-screw extruder and carries out the kneading of the moltenfluoropolymer using paddles mounted upon a shaft for at least 10minutes, 40 to 60 min. being used in the Examples in the presence offluorine. During this melt-kneading, the molten polymer is exposed toreactants which are disclosed to achieve the stabilization anddecolorizing effects, such reactants being at least one of thefollowing: fluorine, water or steam, salts or bases or alcohol.

[0006] European Patent Publication 1 170 303 A1 criticizes the meltkneading of WO 98/09784 as promoting the depolymerization of theperfluorovinyl end groups and deterioration of the fluoropolymer. EP'303 also criticizes the large size and long time required by a surfacerenewal type kneader. EP '303 addresses this problem by melt kneadingthe fluoropolymer in a twin-screw extruder equipped with a kneadingblock in a treatment zone within the extruder for a period of time of0.2 to 5 min, 2 min. being used in the Examples. Both water and oxygenare the reactants added to the treatment zone, the omission of oxygengiving a poor result as indicated in Comparative Example 1. Preferably asalt, potassium carbonate is exemplified, is also present in thetreatment zone. The amount of oxygen present is at least stoichiometricwith respect to the perfluorovinyl groups present, preferably an excessamount, disclosing at least 10 moles of oxygen per mole ofperfluorovinyl group, and particularly a molar excess of 50 to 500. Themolar amount of water present can be the same number of molecules as theunstable end groups to be stabilized, but preferably an excess amount,particularly at least 10× excess amount. To accomplish the stabilizationreported in Example 1 with 2 min. of melt kneading at superatmosphericpressure, the molar excess of water and oxygen relative to —COOH endscalculate to 1337× and 198×, respectively. To accomplish thestabilization reported in Example 2 at subatmospheric pressure, themolar excess of water and oxygen calculate to 853× and 79×,respectively. The stabilization goal as reported in Tables 1 and 2 is toconvert —COOH end groups to —CF₂H.

[0007] EP '303 also discloses fluorine treatment of the fluoropolymerafter the stabilization treatment of the invention of (p. 5, I. 17-19),i.e. after the stabilized fluoropolymer exits the twin-screw extruder.Fluorination provides the end groups of greatest stability, namely —CF₃.

[0008] More efficient and more effective processing of fluoropolymer tochange its character, e.g. to stabilize the fluoropolymer, is needed.

SUMMARY OF THE INVENTION

[0009] The present invention provides a more effective and moreefficient process for stabilizing melt-processible fluoropolymers aswell as for carrying out chemical reactions with fluoropolymers ingeneral. One discovery of the present invention is that in thestabilization treatment of molten fluoropolymer with water reactant toform —CF₂H end groups, oxygen is not a necessary reactant, and in factcan have an adverse result on fluoropolymer quality, and furthermorethat the resultant stabilized fluoropolymer does not discolor during thereaction and therefore does not require subsequent fluorine treatment toremove discoloration. Another discovery of the present invention is thatwhen the desired stabilization effect is the formation of —CF₃ endgroups, the fluorination can be carried out on molten fluoropolymer inmuch less time than the minimum of 10 min in EP '673 and can be carriedout in a twin-screw extruder and not after processing in such extruderas in EP '303.

[0010] The process of the present invention can be described as aprocess for carrying out a chemical reaction with fluoropolymer,comprising

[0011] (a) melting said fluoropolymer,

[0012] (b) contacting said molten fluoropolymer with reactant inisolation from said melting, said contacting being carried out in areaction zone having free volume, and

[0013] (c) subdividing said molten fluoropolymer in said reaction zoneto enable said reactant to effectively contact said molten fluoropolymerso as to carry out the chemical reaction between said reactant and saidmolten fluoropolymer,

[0014] (d) devolatilizing the resultant molten fluoropolymer inisolation from (b) and (c), and

[0015] (e) cooling the devolatilized fluoropolymer.

[0016] Thus, the process of the present invention is a manipulativeprocess for efficiently and effectively carrying out reactions withfluoropolymer. The formation of —CF₂H and —CF₃ end groups are two amongmany different end groups or reaction results that can be achieved bythe process of the present invention, as will be described in greaterdetail later herein.

DETAILED DESCRIPTION

[0017] The zone in which the melting of the fluoropolymer occurs, andthe reaction zone in the process of the present invention are separatefrom one another, enabling the process to be carried out continuously,i.e. as the polymer is melted, it can enter the reaction zone andadvance through the reaction zone to process steps (d) and (e).Preferably, the process is carried out in an extruder, such as atwin-screw extruder, typically having a relatively low usable volumeratio. In accordance with the present invention, however, the melting ofthe fluoropolymer occurs in the melting zone of the extruder, and thismelting zone is isolated from the reaction zone by the moltenfluoropolymer completely filling the usable volume of the extruder. Theusable volume of the extruder is the volume of the extruder barrel thatis not occupied by the extruder screw within the barrel. Thus, themolten polymer forms a “plug” or “seal” within the extruder barrel, atthe transition between the melting zone and the reaction zone. In thereaction zone, however, the usable volume is not completely filled withpolymer. Sufficient volume remains, referred to herein as free volume,to enable the reactant to have open space, i.e. space unoccupied by themolten fluoropolymer and screw elements that accomplish the subdividingof the molten fluoropolymer in the reaction zone.

[0018] The subdividing of the molten fluoropolymer in the reaction zonerapidly regenerates new surfaces of molten fluoropolymer, making thedesired reaction sites available at the surface of the fluoropolymer,thereby minimizing the need for mass transfer to occur in order for thereactant to have access to the reacting site. Thus, the effectivecontact between reactant and molten fluoropolymer occurring in thereaction zone is that which enables the reaction rate to be primarilygoverned by the rate of reaction between the reactant and the unstableend group (reacting site) rather than the by the rate of diffusion ofthe reactant to the end group. This surface regeneration can be achievedby repeatedly subdividing the molten fluoropolymer. The combination offree volume enabling the reactant to circulate within the reaction zonewithout having to depend on mass transfer to reach reaction sites andsubdivided molten fluoropolymer enables this effective contact to occur,wherein the speed of the overall reaction is essentially the rate atwhich the reaction between the reactant and the reacting site proceeds,i.e. essentially not limited by requirement for mass transfer of thereactant from the surface of the molten fluoropolymer into the interiorthereof in order to complete the chemical reaction.

[0019] By way of example of the effectiveness of the contact betweenreactant and molten fluoropolymer in the reaction zone, the instabilityof the fluoropolymer arising from —COOH end groups is removed withoutdiscoloration of the fluoropolymer. Decarboxylation at this unstablesite leaves —CF₂ ³¹ (fluorocarbon anion) end groups in place of the—COOH end groups. Such anionic structures are extremely short-lived inthe fluoropolymer melt, and if no reactant is present in the immediatevicinity of the end, it will quickly lose a fluoride anion, causing theformation of unstable —CF═CF₂ (perfluorovinyl) end groups. The effectivecontact between water reactant and the molten fluoropolymer in thereaction zone in accordance with the present invention, converts the—CF₂ ³¹ to the more stable —CF₂H end group before the reaction to formthe perfluorovinyl end group can occur, i.e. avoiding the formation ofthe unstable perfluorovinyl end groups and avoiding the depolymerizationproblem associated with such groups as disclosed in EP 1170303 A1. Thesame (equivalent) effective contact is present in the subdividing step(c) when the end group is other than —COOH and the reactant is otherthan water, even though different reactions are occurring.

[0020] The subdividing of the molten polymer is simultaneous andrepetitive. Thus, the molten fluoropolymer is subdivided into a largenumber of separate streams at one time in the reaction zone and thissubdividing is repeated a plurality of times within the reaction zone toexpose new surfaces of molten fluoropolymer to the reactant. Kneadingblocks subdivide molten polymer into at most three portions for bilobalgeometries or five portions for trilobal geometries. The subdividingused in the present invention preferably divides the molten polymer intoat least six portions, preferably at least 8 portions, and thissubdivision is repeated at least twice within the reaction zone. Suchsubdivision can be achieved in a twin-screw extruder by using gear orturbine type mixing elements positioned along the extrusion screwswithin the reaction zone. Examples of such mixing elements are the SME,TME, and ZME screw elements that are commercially available fromCoperion Corporation, wherein the screw flights contain interruptions inthe form of notches around their peripheries, at least 6 interruptionsin one rotation of each periphery. The ZME element is described in U.S.Pat. No. 5,318,358 and is depicted as multiple elements in FIG. 4. Asshown in FIG. 1, the ZME elements are reverse pumping with respect tothe polymer being advanced through the extruder by the extrusion screw.Thus, the ZME elements pump the molten polymer backwards(countercurrent) towards the feed end of the extruder, while the notches(slots) in the periphery of the elements permit small streams of moltenpolymer to advance forward through the slots, thus obtaining subdividingof the molten polymer into small portions, at least 10 for each ZMEelement shown in FIG. 4, which can be used in the present invention. TheSME elements resemble the ZME elements but pump the molten polymerforward, while the slots in its periphery cause small streams of moltenpolymer to be formed in a countercurrent pumping action. The TMEelements are neutral with respect to pumping action, i.e. they resemblea gear, whereby the neutral flight of this element tends to hold up flowof molten polymer, while the peripheral slots permit small streams ofmolten polymer to pass through the TME element. These elements can beused in succession within the reaction zone to accomplish the surfaceregeneration necessary for reaction with the reactant to occur withoutundesired side reaction or polymer degradation. Only one of these typesof elements need be used in the succession of elements, or combinationsthereof can be used. They can be separated by forward pumping conveyingelements to provide the desired free volume within the reaction zone.Other types of mixing elements can be used in combination with theseelements or in place thereof, such as mixing elements containing pins orstuds extending from the extruder screw, which disrupt the flow ofmolten polymer, thereby exposing new surface of the polymer for reactionwith reactant, such as shown in FIGS. 2A, and 2D of U.S. Pat. No.5,932,159. It is also possible to put the pins or studs in the barrel ifa corresponding relief is made in the screw, such as in a Buss Kneader®produced by Coperion Corporation. Another alternative that can be usedis cavities or reliefs cut into either/or the screw channel and theextrusion barrel as shown in FIG. 2F of U.S. '159. The interruptions anddisruptions of the molten fluoropolymer in the reaction zone represent asubdividing of the fluoropolymer, including the recombining of themolten fluoropolymer, this occurring a plurality of times within thereaction zone, each time exposing new surfaces of molten fluoropolymermaking them accessible to the reactant.

[0021] The subdividing occurring in the reaction zone is more analogousto distributive mixing than dispersive mixing. In distributive mixing,two or more molten polymers having similar melt viscosities are mixedtogether using equipment that accomplishes the mixing using relativelylow shear. In contrast, equipment used for dispersive mixing subjectsmolten polymer to high shear to break down polymer agglomerates, such asgel particles. The high shear associated with dispersive mixing has thedisadvantage of excessively degrading the molten fluoropolymer, creatingunstable end groups and discoloration if too much dispersive mixing isused. U.S. Pat. No. 5,932,159 discloses and depicts a variety ofdistributive mixing devices and dispersive mixing devices, and describesthe use of kneading blocks to accomplish dispersive mixing (col. 3, I.47-48). While the subdividing carried out in the process of the presentinvention is like distributive mixing, such subdividing provides adifferent result, i.e. a regeneration of surface area to minimize thedistance required for mass transfer and allow rapid and efficientchemical reaction between the reactant and the molten fluoropolymer,rather than the mere mixing together of different polymers.

[0022] Notwithstanding the extensive subdividing of the moltenfluoropolymer in the reaction zone and including even the possibility ofportions of the molten fluoropolymer repetitively including intervals ofcountercurrent flow, the reaction zone has free volume within the entirereaction zone or within portions thereof. In an extruder, such as atwin-screw extruder, the reaction zone will be a continuation of themelting zone, i.e. the extruder barrel will have the samecross-sectional area in both zones, but the reaction zone will beseparated from the melting zone by a melt plug and will contain freevolume. The melt plug that separates the zones can be achieved by anumber of techniques including, but not limited to, utilizing a reversepitch element, utilizing an element that restricts polymer flow,reducing the cross-sectional free area, and/or using a diminished pitchelement at the end of the melting zone. The creation of free volumedownstream from the melt plug and within the reaction zone is normallyachieved by increasing the free volume to a volume greater than thevolume of the melt in the reaction zone. This can be achieved throughseveral techniques with the most common being to increase the forwardpitch of the element, but it can also be achieved by changing thecross-sectional geometry of the screw(s) to increase the free area.Either technique or the combination thereof can be used in the practiceof the present invention.

[0023] In greater detail, with respect to process step (a), the meltingof the fluoropolymer can be carried out either in the same extruder thatis used for the reaction or in a separate device. The use of the sameextruder for both melting and reaction is usually the most economicaland will be exemplified herein. If the melting occurs in the sameextruder as the reaction, the melting would normally occur byconventional means by the application of sufficient heat, usually atleast about 40° C. greater than the melting temperature of thefluoropolymer, and mechanical energy input for sufficient time to causethe fluoropolymer to become molten. Since molten fluoropolymer generallyhas a high viscosity, it is preferred that the temperature of the meltis at least about 30° C. above the melting point of the fluoropolymer.The melting point of the fluoropolymer is the peak of the endothermobtained using the thermal analyzer in accordance with the procedure ofASTM 3159. When the process is carried out in an extruder, theconfiguration of the screw or screws, if the extruder is a twin-screwextruder, can be conventional to convey the fluoropolymer from the feedend of the extruder towards the reaction zone, while the melting of thefluoropolymer occurs from the input heating and the mechanical energyfrom the extrusion screw advancing the fluoropolymer through the meltingzone. The conveyance of the molten fluoropolymer through the meltingzone continues into the reaction zone, whereupon the screw elementspresent in the reaction zone, while accomplishing the subdividingdescribed above, continue the conveyance of the molten fluoropolymerthrough the reaction zone and towards the outlet of the extruder.

[0024] With respect to process step (b) the contacting of the moltenfluoropolymer with reactant occurs preferably by feeding the reactantinto the reaction zone independently of the feed of the moltenfluoropolymer into the reaction zone. This, together with the isolationof the reaction zone from the melting zone, enables the amount ofreactant to be controlled and isolated from the feed end of theprocessing equipment. Depending on the particular reactant used, thisisolation is also important for safety reasons, by preventing thereactant from leaking from the reaction zone to the feed end of theprocessing equipment, and then to the atmosphere, unless the feed end ofthe processing equipment is kept isolated from the atmosphere.

[0025] The feed of the reactant or reactants can come from a single ormultiple inlet ports into the reaction zone and the reactant ispreferably in the form of a fluid, which at reaction temperature, i.e.the temperature of the molten fluoropolymer, can be at, above, or belowthe critical temperature of the reactant, and can be a gas or a liquidor a solid that decomposes to a gas. The temperature of the reactionzone, which will be at least the temperature of the molten polymerentering the reaction zone from the melting zone, can, but does not haveto, cause the reactant to change phase (vaporize or melt) or tochemically change (decompose). The process of the present invention hasa number of preferred conditions, which can be used separately ortogether depending on the particular reactant being used. For example,while oxygen can be a reactant to convert carbon contaminant in thefluoropolymer to CO₂ to improve color, oxygen is not a necessaryreactant in the conversion of —COOH end groups to the more stable —CF₂H.A proton (hydrogen) source, such as water, is an essential reactant toform the more stable hydride end group. Large excesses of reagentsorders of magnitude above the stoichiometric level such as the water asused in EP '303 are indicative of a process with either mass transfer orkinetic limitations. The large excess of water has several disadvantagesincluding leading to excessive corrosion and generation of liquid waste.Even with the excessive water, the reaction is still not efficientenough to convert the fluorocarbon anion that results fromdecarboxylation directly to the stable hydride. Instead, many of thefluorocarbon ions lose a fluoride anion to form the —CF═CF₂(perfluorovinyl) end group. To eliminate the —CF═CF₂ end groups, oxygenhas to be added to convert the perfluorovinyl end groups to acidfluoride —COF end groups, which in turn react with the water reactant toform —COOH groups, which continue to react through the above cycle withonly a portion of the ends reacting with the water to form the stablehydride end groups in any cycle. Thus, poor efficiency of contactbetween the polymer end groups and water promotes cycling of theendgroup reactions through multiple formations of the starting —COOH endgroup, and requires oxygen being an essential reactant to convert theperfluorovinyl end group to —COF, so as to be convertible back to —COOH.A large enough excess amount of water reactant eventually succeeds inconverting most of the —COOH (the —COOH is decarboxylated), initiallypresent, and interimly formed, to —CF₂H. The requirement for oxygenreactant in the process of EP '303 indicates the formation of carbon,being caused by polymer degradation and by depolymerization of unstableperfluorovinyl end groups, the formation of the latter being anindicator of insufficient contact between water reactant and moltenpolymer, whereby the conversion of fluorocarbon anion to perfluorovinylend groups occurs before hydride end-capping to form —CF₂H, by virtue ofthe delay caused by the need for mass transfer for the water reactant toreach the decarboxylation reaction site.

[0026] The much greater efficiency of contact between reactant andmolten fluoropolymer in the present invention enables the hydrideend-capping to be carried out without feeding oxygen into the reactionzone as a reactant separate from the feed of the fluoropolymer into thetreatment zone in EP '303. Oxygen does not have to be excluded from thereaction zone however, in the water/—COOH reaction system, which meansthat the fluoropolymer feed to the processing equipment, such feedgenerally being in the form of flakes, does not have to be deoxygenatedto rid the surface of the flakes of absorbed oxygen. Even this smallamount of absorbed oxygen is mostly flushed out of the moltenfluoropolymer, back towards the feed end of the extruder, by the meltingprocess, whereby the amount of oxygen, if any, reaching the reactionzone is much less in molar amount than the moles of —COOH end groupspresent. Thus, the absence of any feed of oxygen into the reaction zoneseparate from the feed of fluoropolymer into the reaction zone isconsidered as carrying out the reaction essentially free of oxygen.Because of the ability to rapidly regenerate surface area and eliminatemass transfer limitations, the reaction can quickly and completelyproceed with a molar amount of reactant that is less than 10×,preferably less than 5×, with respect to the molar amount of thereactive moiety in the molten fluoropolymer, either end groups orcontaminant present in the fluoropolymer, or both. In the reactioninvolving water and —COOH end groups, it is even possible to use fewermoles of reactant than moles of —COOH end groups present to obtainreplacement of the —COOH end groups by —CF₂H end groups, by virtue ofsmall amounts of hydrogen being available from the hydrogen of the —COOHend groups. As stated above, in the case of the water/—COOH reactionsystem (—COOH end groups on the molten fluoropolymer), the excess ofwater should be such as to cause formation of hydride end groups,whereby oxygen does not have to be added to the reaction zone. Excellentresults are obtained even when the molar amount of water reactant isless than the moles of —COOH end groups present in the polymer. Thepossible presence of oxygen absorbed on the surface of the fluoropolymerflakes fed to the processing equipment is not addition of oxygen to thereaction zone in the sense that the step of addition is an action thatis independent of the feeding of the flake to the processing equipment.

[0027] A wide variety of reactions involving the molten fluoropolymerare contemplated by the present invention, such as follows:

[0028] (i) the end groups are acid end groups, —COF or —COOH and thereactant is ammonia, to form the more stable amide end group —CONH₂;

[0029] (ii) the end groups are acid end groups, —COF or —COOH, and thereactant is a primary or secondary amine, such as dimethyl, diethyl orpropyl amine, to form amide end groups —CONRH or —CONR₂, wherein R isthe alkyl group(s) of the amine, where for R₂, the alkyl groups are thesame or different;

[0030] (iii) the end groups are acid end groups, —COF or —COOH and thereactant contains an alcohol, such as methanol, ethanol, propanol, or afluorine containing alcohol to form the more stable ester —COR′ where R′is the alkyl group supplied by the alcohol;

[0031] (iv) the reactant contains fluorine to convert such end groups as—COOH, amide, hydride, —COF, and other nonperfluorinated end groups orperfluorovinyl to —CF₃ end groups;

[0032] (v) The reactant contains an oxygen or fluorine to convertoxidizable contaminant to a gaseous compound, e.g. carbon, to CO₂ orCF₄; and

[0033] (vi) the end groups are acid end groups, —COF or —COOH, orcarboxylate salt end groups, preferably alkali metal carboxylate salt,and the reactant is a proton source to form the more stable —CF₂H endgroup.

[0034] These reactions are carried out to the degree of completiondesired, depending on the reaction and the intended use of thefluoropolymer.

[0035] With respect to reactions (iv) and (v), the fluorine can be inthe form of fluorine gas, which may be diluted with nitrogen, argon,helium, krypton, neon, xenon, carbon dioxide, or other inert gas or canbe in the form of a fluorinating agent such as any of the fluoridecompounds disclosed in British Patent 1,210,794, which supply fluorineunder reaction conditions. Dilution of fluorine with an inert gas isadvisable to avoid excessive or excessively rapid fluorination in thereactor. Excessive reaction can result in overheating and/oroverpressuring, a hazardous condition. At concentrations of 20 mol % orgreater, fluorine can spontaneously exothemically react withhydrocarbons. Although higher than 20% concentrations of fluorine can beused with fluoropolymers, the fluorine concentration is preferably nomore than 15 mol % to provide sufficient safety factor in case ofaccidental hydrocarbon or water contamination. The oxygen reactant usedin reaction (v) can be in the form of oxygen gas, in neat or dilutedform, or can be in the form of an oxidizing agent such as, but notlimited to, peroxides, chlorates, perchlorates, nitrates, andpermanganates, which make oxygen available during the reaction. Withrespect to reaction (vi), the proton source is hydrogen, which is in theform of H⁺, to react with the —CF₂ ⁻ remaining after decarboxylation.This proton source as the reactant is described herein primarily aswater because of water being the most economical source for the protonunder the conditions of the reaction. The water can be added to thereaction zone as a liquid, whereupon it will instantly become steam, ormay be added as steam to the reaction zone. Wherever water is disclosedas the reactant, such water can also be considered as the proton source.

[0036] These reactions, (i)-(iv) and (vi), are not limited to thestarting end groups specified. Where a plurality of end groups arespecified they all may be present or other non-specified end groups mayalso be present.

[0037] With respect to process step (c), the subdividing occurringtherein is described above. With respect to the reaction time, which isthe residence time of the molten fluoropolymer in the reaction zone, thereaction can be carried out essentially to completion in no more than 5minutes, preferably no more than 120 seconds, and more preferably nomore than 60 seconds. The residence time in the reaction zone accordingto this invention is determined as follows: The ZME elements areconsidered to be fully filled with polymer and the residence time, Θ, inthe ZME elements is Θ=V/q, where V is the useable volume of the elementand q us the volumetric throughput rate. For the conveying bushings,which are not fully filled, Θ=2L/ZN, where L is the length of thereaction zone that contains the bushings, Z is the pitch (the axialdistance required for a single rotation of the screw), and N is thescrew speed. The residence times for the ZME elements and the bushingsare summed to obtain the total residence time in the reaction zone. The1 mm spacers are ignored. They comprise only about 1% of the length ofthe reaction zone in the Examples disclosed herein.

[0038] The combination of short reaction time and subdividing such as bythe distributive type of mixing of the molten fluoropolymer in thereaction zone enables the reaction to be completed in a short timewithout causing undesirable side reactions, such as occur uponsubjecting the melt to high shear as in the dispersion type mixing orupon prolonged exposure of the melt to the high melt temperatures.Fluoropolymer processed according to the present invention has excellentcolor with a whiteness index normally above 60. In U.S. '587, polymercolor is measured by the reflectance of green light, a high %Gindicating a high level of whiteness. The whiteness index, described indetail before the Examples, provides an improved determination ofwhiteness. For example, the whiteness index of at least 60 for FEPprocessed by reaction (vi) above is as good a white color as the %Gvalues reported in Table IV of U.S. '587 after fluorination of thepolymer.

[0039] The reaction temperature in the reaction zone will typically beno more than about 400° C. and at least about 30° C. greater than themelting point of the fluoropolymer, typically at least about 300° C.,applicable to lower melting fluoropolymers such asethylene/tetrafluoroethylene copolymer.

[0040] With respect to steps (d) and (e), to remove the volatileproducts of reaction, any unreacted reagents, and any inerts, one ormore removal ports are provided. In extruders, these are known as vacuumports or vent ports, positioned at the end of the reaction zone. A meltseal is preferably provided between the reaction zone and the removalport to isolate the devolatilization from the reaction zone. Such meltseal can be provided by known means. When the reaction is carried out inan extruder, the extruder screw element(s) creating the melt seal (plug)can either be reverse pumping, of diminished forward pitch, orpossessing increased screw volume so as to accomplish the seal. Prior tothe removal port, an inert gas or gases may be added to the moltenpolymer to facilitate removal of unreacted reagents or reactionproducts. This is a process known as stripping that is widely reportedin the art.

[0041] With respect to process step (e), the molten fluoropolymer isoften passed through a melt filter and then a die to achieve the finaldesired shape. Depending on the pressure drop through these operations,the process might employ an additional pump that could take the form ofanother extruder (normally a single-screw extruder), a gear pump, orother pumping device. The die can be designed to produce a finished orsemifinished product. Some examples include film, profiles, tubes, wirecoatings, fibers, and other objects. The die can also be designed toproduce pellets or cubes. The later is achieved by either extruding astrand and chopping it or melt cutting. Once the product leaves the die,it is rapidly cooled to form a solid that maintains the desired shape.

[0042] Additionally, it is preferred that the fluoropolymer aftercooling be sparged so as to remove decomposition products, if any, thatremain in the fluoropolymer upon solidification. Sparging is normallyaccomplished by passing a gas (air and nitrogen are the most commongases) around the extruded product. The gas can be at ambient conditionsor heated to a temperature at least about 10° C. below the melt point.Higher temperatures tend to allow shorter sparge time. The normal rangeof sparge temperatures are between about 25° C. and 250° C. with spargetimes of between one hour and twenty-four hours, but the exactconditions can be varied to insure adequate removal of dissolved gases.

[0043] One group of fluoropolymers used in the process of this inventionare characterized by having at least about 90%, preferably at leastabout 95%, and more preferably all of the monovalent atoms as halogens,preferably as fluorine. That is, the polymers are preferablyperhalogenated, more preferably perfluorinated. The polymers may becrystalline or amorphous. By crystalline is meant that the polymers havesome crystallinity and are characterized by a detectable melting pointmeasured according to ASTM D 3418, and a melting endotherm of at leastabout 3 J/g. Melt-processible polymers that are not crystallineaccording to the preceding definition are amorphous. Amorphous polymersinclude elastomers, which are distinguished by having a glass transitiontemperature of less than about 20° C. as measured according to ASTM D3418. By the term “melt-processible” it is meant that the polymer can beprocessed (i.e., fabricated into shaped articles such as films, fibers,tubes, wire coatings and the like) by conventional melt processingmeans. Such processibility requires that the melt viscosity at theprocessing temperature be no more than 10⁶ Pa·s. Preferably it is in therange of about 10² to 10⁶ Pa·s, and most preferably about 10⁴ to 10⁶Pa·s. Another group of melt-processible fluoropolymers that can be usedin the process of the present invention are those that containhydrocarbon groups in the polymer chain, with the fluoropolymernevertheless containing at least about 35 wt % fluorine.

[0044] Melt viscosities of the melt-processible perfluoropolymers weremeasured according to ASTM D 1238-52T, modified as follows: Thecylinder, orifice and piston tip are made of a corrosion-resistantalloy, Haynes Stellite 19, made by Haynes Stellite Co. The 5.0 g sampleis charged to the 9.53 mm (0.375 inch) inside diameter cylinder, whichis maintained at 372° C.±1° C., such as disclosed in ASTM D 2116 andASTM D 3307 for perfluorinated polymers. Five minutes after the sampleis charged to the cylinder, it is extruded through a 2.10 mm (0.0825inch) diameter, 8.00 mm (0.315 inch) long square-edge orifice under aload (piston plus weight) of 5000 grams. This corresponds to a shearstress of 44.8 kPa (6.5 pounds per square inch). The melt viscosity inPa·s is calculated as 53170 divided by the observed extrusion rate ingrams per 10 minute. The melt viscosity of fluoropolymers containinghydrocarbon groups in the polymer chain can be determined in accordancewith ASTM procedures for these particular polymers, such as ASTM D 3159and ASTM D 5575.

[0045] Examples of melt processible polymers used in this invention arepolychlorotrifluoroethylene and copolymers of tetrafluoroethylene (TFE),with one or more of comonomers such as hexafluoropropylene (HFP),perfluoro(alkyl vinyl ether) (PAVE), and perfluorodimethyl dioxole(PDD). TFE/HFP copolymers are commonly known as FEP. TFE/PAVE copolymersare commonly known as PFA or MFA. PAVE include perfluoro(alkyl vinylether), wherein the alkyl group contains from 1-8 carbon atoms,preferably 1 to 3 carbon atoms, such as perfluoro(propyl vinyl ether(PPVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinylether) (PMVE) or mixtures thereof. An additional example ofperfluoropolymer are the elastomers as tetrafluoroethylene/vinylidenefluoride copolymer, optionally containing hexafluoropropylene, andtetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymers, alkyl beingdefined as above. Examples of fluoropolymers containing hydrocarbongroups in the polymer chain are copolymers of tetrafluoroethylene orchlorotrifluoroethylene with ethylene and copolymers oftetrafluoroethylene/hexafluoropropylene/vinylidene fluoride, known asTHV. The copolymers described above can include one or more additionalcomonomers in small amounts to improve the properties of thefluoropolymer. Endgroups are measured by the measured by the methodsdescribed in U.S. Pat. Nos. 3,085,083 and 4,675,380.

[0046] Reaction process (vi) carried out with FEP, not only stabilizesthe polymer backbone, by ridding the polymer of HFP diads and triads(adjacent HFP units), but also stabilizes the end groups by convertingthem to —CF₂H end groups, with subsequent fluorination being unnecessaryto improve polymer color. The ridding of HFP diads and triads isprimarily a rearrangement of atoms, rather than formation of new endgroups, as indicated by little change in molecular weight occurring inthe backbone stabilization process. The FEP reacted with water inaccordance with the present process provides an FEP which has excellentcolor as indicated by a whiteness index of at least 60 and which isbackbone stabilized. The process of the present invention does notpromote degradation of the FEP polymer while the backbone stabilizationis occurring. After sparging, such as by heating in an air oven fortwelve hours at 150° C., to remove dissolved gases, the FEP exhibits abackbone volatiles index of less than 22 and a total volatiles index ofless than 30, indicating stabilization of the polymer backbone as wellas the end groups. Except for this backbone stabilization of FEP andreaction (v) above, the process of the present invention is generallydirected at the most reactive moieties present in the normallynon-reactive, chemically inert fluoropolymers, namely the end groups ofthe fluoropolymer. Thus, the composition of the fluoropolymer itself,i.e. excluding end groups, is generally unimportant as not participatingon the reaction. Preferably, the fluorine treatment of reaction (iv) iscarried out on perfluorinated polymers that contain unstable end groups,so that the reaction occurs primarily with these end groups.

[0047] Volatiles index is a measure of the stability of thefluoropolymer by subjecting the fluoropolymer to heating at a hightemperature for a certain period of time and determining the bubbleformation occurring as a result of such heating. The procedure ofdetermining volatiles index as reported herein is similar to the methoddisclosed in U.S. Pat. No. 4,626,587, except that the pre-heat treatmentis not used, the polymer sample size is 18.5 g, the volume of theapparatus is 124 ml, the temperature of heating of the polymer is 360°C., and the equation used is:

Backbone volatiles index=P40−P10

Total volatiles index=P40−P0

[0048] wherein P0, P10 and P40 are the pressures of the sample in mm Hgafter 0, 10 and 40 minutes in the hot block. The pressure of thevolatiles released (bubble formation) from the polymer at 10 min and at40 min heating time is compared to obtain the backbone volatiles index.The total volatiles index is calculated in the same fashion as thebackbone volatiles index, but it uses the pressure of the volatilesreleased from the polymer at 0 min and at 40 min heating time. So thatthe determination of volatiles index is not skewed by the presence ofdissolved gases present in the fluoropolymer after practice of theprocess of the present invention, the fluoropolymer is subjected toeither a commercial sparging process as described above, or in the caseof laboratory samples, the following heat treatment prior to thedetermination of volatiles index: the fluoropolymer is heated (sparged)in an air oven at 150° C. for 12 hours. The total volatiles index offluoropolymers processed in accordance with the present invention is nogreater than 30. The backbone volatiles index is no greater than 22.This backbone volatiles index of 22 is comparable to the value of 10quoted in U.S. Pat. No. 4,626,587 as being the limit for bubble freeproduct after adjusting for the differences in test equipment and testconditions. These volatiles indices may be exhibited by TFE/PAVEcopolymers without the stabilization treatment in accordance withreaction (iv), by virtue of fewer unstable end groups being present thanwith respect to TFE/HFP copolymer and the lack of instability associatedwith HFP diads in TFE/HFP copolymers. These TFE/PAVE copolymers areadequate for some applications, but they can show deficiencies in otherapplication for issues including, but not limited to, color formation,HF evolution, and extractable ion content, which can be eliminated byexposure of the copolymer to reaction (iv).

[0049] The whiteness index is determined by using a HunterlabTristimulus Colorimeter, Model D25M-9 in accordance with theinstructions provided with the Colorimeter. The determination involvesfilling the optically clear cylindrical sample holder (6.35 cm innerdiameter) with cubes of the polymer, exposing the sample holder to whitelight (light source provided with the Colorimeter) circumferentially,and measuring the reflected light using four silicon diode detectors.Preceding this determination, the Colorimeter is calibrated using astandard whiteness plaque supplied by the Colorimeter manufacturer.

[0050] The whiteness index of at least about 60 achieved by subjectingFEP to reaction (vi) is also preferably achieved by each of the otherreactions on each of the other fluoropolymers. The process of thepresent invention does not cause color formation by polymer degradation.The volatiles index and whiteness index can vary independently. Thus,the fluoropolymer can have a low volatiles index and exhibit poor color,well below the desired minimum whiteness index of about 60.

[0051] Preferably the reaction (iv) is carried out so that the sum ofthe amount of the end groups —COOH, —COF, and —CONH₂ is less than about80/10⁶ carbon atoms, more preferably less than about 30/10⁶ carbonatoms, and most preferably, less than about 6/10⁶ carbon atoms.

EXAMPLES

[0052] The whiteness index values and volatiles index values reported inthe experiments below are determined as described above. The volatilesindex is determined after the polymer has been sparged at the specificconditions mentioned above so that the dissolved volatiles do not maskthe stabilization of the polymer measured by exposure to much highertemperatures.

[0053] All of the experiments are carried out with a 57 mm twin-screwextruder, equipped with an injection probe, which is a rod having alongitudinal bore opening flush with the surface of the extruder barrelin the reaction zone, and a vacuum port connected to afluorine/hydrofluoric acid scrubbing system, which feeds into a 120 mmsingle-screw extruder, which is equipped with a die. The twin-screwextruder serves as a resin melter and end group reactor in which thedesired end group, and in the case of FEP, backbone, stabilization isconducted. The single-screw extruder serves as a melt pump to generatethe pressure necessary to move the resin through the optional screenpack and die.

[0054] The extrusion equipment described above is a “Kombiplast”extruder from the Coperion Corporation. Corrosion-resistant materialsare used for those parts that come into contact with the polymer meltand fluorinating agent. The twin-screw extruder has two corotatingscrews disposed side by side. The screw configurations are designed withan intermeshing profile and tight clearances, causing them to beself-wiping. The screw configurations include kneading blocks, mixingelements, and conveying screw bushings. The first 15 Length/Diameter(L/D, D being the diameter of the bushings) of the extruder is themelting zone. This contains the feeding, solids conveying, and kneadingblock sections. The kneading block sections provide high shear andinsure proper melting of the polymer. The melting section ends with aleft handed bushing (rearward pumping) that forms a melt seal andinsures complete filling of the final kneading blocks.

[0055] The reagent is injected immediately after this section. The next11 L/D contain the injection, mixing and reaction sections with multiplemixing elements and constitute the reaction zone of the extruder. Themixing elements used and their arrangement for one effective design usedin Examples 1-8 is as follows: two ZME elements, two 40 mm conveyingbushings (40 mm is both the length of the element and the pitch of thehelical flight in one revolution of the element), a single ZME element,two 40 mm conveying bushings, two ZME, two 40 mm conveying bushings, anda left handed bushing to provide a melt seal with respect to thedevolatilization zone. The mixing elements used in Example 10 and theirarrangement for another effective design is as follows: two ZMEelements, one 40 mm conveying bushing (40 mm is both the length of theelement and the pitch of the helical flight in one revolution of theelement), three ZME elements, one 40 mm conveying bushing, two ZME, one40 mm conveying bushing, two ZME, one 40 mm conveying bushing, one 30 mmconveying bushing and a left handed bushing to provide a melt seal withrespect to the devolatilization zone. A 1 mm thick spacer ring ispresent between conveying bushings and ZME elements. The next 5 L/Dcontains the vacuum extraction section (devolatilization zone), which isconnected to a scrubbing system designed to neutralize F₂, HF, and otherreaction products, depending on the reaction being carried out. Thevacuum extraction section follows a conventional design, which includesmelt forwarding elements that provide for free volume, so that themolten polymer is exposed to subatmospheric pressure so that reactiveand corrosive gases do not escape into the atmosphere. The vacuum can beoperated between 0.2 and 14.6 psia (1.4-101 kPa absolute), morepreferably between 5 and 14 psia (34-97 kPa absolute), and mostpreferably between 8 and 13 psia (55-90 kPa absolute). In the Examples,the vacuum is about 10 psia (69.2 kPa absolute). Undercut bushings (SK)are an effective way to provide the forwarding elements in the vacuumextraction section of the extruder and these are used in the vacuumextraction section in the extrusions described in the Examples. Thefinal 2 L/D are used to provide a vacuum seal and pump the moltenpolymer into the single-screw extruder. End group reactions mainly occurin the section between the injection nozzle and the vacuum port thatcontains the mixing sections. Backbone stabilization in the case of FEPoccurs in both the kneading block sections and the mixing sections.Surprisingly this backbone stabilization, beginning as early as themelting zone is not accompanied by discoloration of the FEP.

[0056] The twin-screw extruder empties into a single-screw melt pump,which is designed to generate pressure at low shear rates for filtrationand pellet formation. The extruded melt is melt cut through a die with250 2.5 mm die holes. The pellets are cooled by a stream of water.

[0057] The twin-screw extruder is operated with barrel temperatures ofabout 300° C. to 380° C. The single-screw extruder is operated withbarrel temperatures of about 300° C. to 350° C.

Example 1 (no reaction)

[0058] A compacted flake of a copolymer of tetrafluoroethylene (TFE) and3.7 weight percent perfluoro(propyl vinyl ether) (PPVE) commonly knownas PFA polymerized with ammonium persulfate (APS) initiator is used asthe feed material. The polymer has an initial melt flow rate (MFR) of14.0. The polymer end groups consist of 459 end groups havingcarbon-hydrogen bonds, divided approximately equally between —CF₂H and—CF₂CH₂CH₃ end groups; 180 —COOH end groups, 0 —COF end groups, and 19—CONH₂ end groups. The ethyl end groups come from the use of ethanechain transfer agent in the copolymerization process producing thecopolymer. With other chain transfer agents, the end groups would bedifferent and characteristic. For example, with methane, the end groupis —CF₂—CH₃.

[0059] The flake is processed through the above extrusion equipment andno reagents are injected into the extruder. The screw speed is adjustedto obtain a residence time of 33 seconds in the reaction zone.

[0060] The extruded pellets have a MFR of 14.1. The polymer end groupsconsist of 377 end groups having carbon-hydrogen bonds, dividedapproximately equally between —CF₂H and —CF₂CH₂CH₃ end groups, 68 —COOHend groups, 38 —COF end groups, and 20 —CONH₂ end groups. The backbonevolatiles index is 8.7 and the total volatiles index is 18.7 and thewhiteness index is 58.38.

Example 2 (the invention)

[0061] The same compacted flake feed as Example 1 is processed throughthe above extrusion equipment and a fluorination agent consisting of 10mole % F₂ in N₂ is injected into the extruder. The ratio of fluorine topolymer is 2500 ppm by weight (1.5× molar excess with respect to all themoles of the non-perfluorinated end groups). The screw speed is adjustedto obtain a residence time of 33 seconds in the reaction zone. Theextruded pellets have a MFR of 15.1. The polymer end groups consist of 0—CF₂H end groups, 0 —CF₂CH₂CH₃ ends, 0 —COOH end groups, 0 —COF endgroups, and 0 —CONH₂ end groups. The backbone volatiles index is 1.9,the total volatiles index is 11.0 and the whiteness index is 76.89.Similar results are obtained when the fluorination agent is 5 mole % F₂in N₂.

Example 3 (no reaction)

[0062] A compacted flake of a copolymer of tetrafluoroethylene (TFE),with 11.6 weight percent hexafluoropropylene (HFP), and 1.3 weightpercent perfluoro(ethyl vinyl ether) (PEVE) commonly known as FEPpolymerized with a mixed ammonium/potassium persulfate (APS/KPS)initiator is used as the feed material. The polymer has an initial meltflow rate (MFR) of 21.9. The polymer end groups consist of 0 —CF₂H endgroups, 599 —COOH end groups, 0 —COF end groups, and 0 —CONH₂ endgroups.

[0063] The flake is processed through the above extrusion equipment andno reagents are injected into the extruder. The screw speed is adjustedto obtain a residence time of 30 seconds in the reaction zone.

[0064] The extruded pellets have a MFR of 23.9. The polymer end groupsconsist of 546 —CF₂H end groups, 0 —COOH end groups, 1 —COF end groups,and 6 —CONH₂ end groups. The backbone volatiles index is 12.6, the totalvolatiles index is 21.4, and the whiteness index is 14.70, which isextremely poor. The processing of the FEP with a salt such as theresidue from the potassium persulfate initiator (from thecopolymerization process) in the extruder causes the shift from —COOHend groups to —CF₂H end groups, but the chemistry is accompanied byformation of carbon as indicated by its poor color. Thus thedecarboxylation of the —COOH end groups is accompanied by otherreactions, including the formation of —CF═CF₂ end groups, which areknown to decompose to produce elemental carbon (and CF₄) upon furtherexposure to heat.

Example 4 (the invention)

[0065] The same compacted flake feed as Example 3 is processed throughthe above extrusion equipment and 145 ppm by weight H₂O as steam isinjected into the extruder (0.7× the moles of —COOH end groups). Thescrew speed is adjusted to obtain a residence time of 30 seconds in thereaction zone.

[0066] The extruded pellets have a MFR of 24.2. The polymer end groupsconsist of 511 —CF₂H end groups, 0 —COOH end groups, 0 —COF end groups,and 16 —CONH₂ end groups. The backbone volatiles index is 12.9 and thetotal volatiles index is 22.9. Color is good, i.e. the whiteness indexis 63.88. The presence of the small amount of water reactant togetherwith the effective contact between the molten FEP and the water in thereaction zone directs the decarboxylation reaction to produce the stablehydride end groups without the accompanying reactions which causediscoloration of the polymer.

Example 5 (comparison with Example 4)

[0067] The same compacted flake feed as Example 3 is processed throughthe above extrusion equipment and 125 ppm by weight H₂O (0.6× the molesof —COOH end groups) and 2288 ppm by weight air (536 ppm oxygen or 1.2×the moles of —COOH end groups) is injected into the extruder. The screwspeed is adjusted to obtain a residence time of 30 seconds in thereaction zone.

[0068] The extruded pellets have a MFR of 25.9. The polymer end groupsconsist of 0 —CF₂H end groups, 50 —COOH end groups, 209 —COF end groups,and 0 —CONH₂ end groups. The backbone volatiles index is 24.8 and thetotal volatiles index is 52.0 and color is poor; the whiteness index is26.83. The addition of the oxygen reactant to the reaction zoneinterfered with the hydride end capping desired from the water reactant,giving both poor volatiles index and color.

Example 6 (the invention)

[0069] The same compacted flake feed as Example 1 is processed throughthe above extrusion equipment and ammonia gas (NH₃) is injected into theextruder. The ratio of ammonia to polymer is 982 ppm by weight (2× themoles of —COOH end groups). The screw speed is adjusted to obtain aresidence time of 33 seconds in the reaction zone.

[0070] The polymer end groups are primarily —CONH₂ end groups. Similarresults are achieved when the ammonia gas is replaced by an equivalentamount of dimethylamine, and the resulting end groups are primarily—CON(CH₃)₂.

Example 7 (the invention)

[0071] The same compacted flake feed as Example 1 except with —COF endgroups resulting from polymerization in a nonaqueous media with aperoxide initiator is processed through the above extrusion equipmentand liquid Zonyl® BA is injected into the extruder. Zonyl® BA has theformula:

R″—CH₂CH₂OH

[0072] Where R″ is

[0073] 1-2% C₄F₉

[0074] 27-34% C₆F₁₃

[0075] 29-34% C₈F₁₇

[0076] 17-21% C₁₀F₂₁

[0077] 6-9% C₁₂F₂₅

[0078] 2-5% C₁₄F₂₉

[0079] 1-2% C₁₆F₃₃,

[0080] all %s being by weight.

[0081] The ratio of Zonyl® BA to polymer is 24,000 ppm by weight (2× themoles of —COF end groups). The screw speed is adjusted to obtain aresidence time of 33 seconds in the reaction zone.

[0082] The polymer end groups primarily consist of —COR″ end groups witha similar ratio of R″ as the Zonyl® BA.

Example 8 (the invention)

[0083] Scrap film of a copolymer of tetrafluoroethylene (TFE) and 3.7weight percent perfluoro(propyl vinyl ether) (PPVE) commonly known asPFA is extruded to form cubes. The cubes have extremely poor color (WIof 0.8). The cubes are processed through the above extrusion equipmentand a fluorination agent consisting of 10 mole % F₂ in N₂ is injectedinto the extruder. The ratio of fluorine to polymer is 3000 ppm byweight. The screw speed is adjusted to obtain a residence time of 33seconds in the reaction zone. The extruded pellets have good color witha whiteness index of 62.

[0084] This experiment is repeated except that air instead offluorinating agent is injected into the reaction zone of the extruder.The ratio of air to polymer is 10% (100,000 ppm) by weight. The screwspeed is adjusted to obtain a residence time of 33 seconds in thereaction zone. The extruded pellets have good color with a whitenessindex of 60.

Example 9 (no reaction)

[0085] A compacted flake of a copolymer of tetrafluoroethylene (TFE),with 12.2 weight percent hexafluoropropylene (HFP), and 1.2 weightpercent perfluoro(ethyl vinyl ether) (PEVE) commonly known as FEPpolymerized with ammonium persulfate (APS) initiator is used as the feedmaterial. The polymer has an initial melt flow rate (MFR) of 33.7. Thepolymer end groups consist of 0 —CF₂H end groups, 728 —COOH end groups,0 —COF end groups, and 28 —CONH₂ end groups.

[0086] The flake is processed through the above extrusion equipment andno reagents are injected into the extruder. The screw speed is adjustedto obtain a residence time of 22 seconds in the reaction zone.

[0087] The extruded pellets have a MFR of 29.1. The polymer end groupsconsist of 0 —CF₂H end groups, 697 —COOH end groups, 0 —COF end groups,and 9 —CONH₂ end groups. The backbone volatiles index is 65.2 and thetotal volatiles index is 84.3 indicating a very unstable polymer that isnot useful for extrusion wire coating. The whiteness index is 62.67.

Example 10 (the invention)

[0088] The same compacted flake feed as Example 9 is processed throughthe above extrusion equipment and a fluorination agent consisting of 10mole % F₂ in N₂ is injected into the extruder at 1100 ppm by weightfluorine. The screw speed is adjusted to obtain a residence time of 22seconds in the reaction zone.

[0089] The extruded pellets have a MFR of 28.7. The polymer end groupsconsist of 0 —CF₂H end groups, 7 —COOH end groups, 2 —COF end groups,and 1 —CONH₂ end groups. The backbone volatiles index is 11.9 and thetotal volatiles index is 21.9. Color is very good, i.e. the whitenessindex is 72.32.

Example 11 (wire coating)

[0090] The improved extrusion wire coating obtained by subjectingfluoropolymer to the fluorination treatment of reaction (iv) is shown inthis Example. The fluorine-treated copolymer of Example 10 is used inthis Example. The improved extrudability of the copolymer is shown byreduced copolymer drool formed at the die orifice, which is periodicallycarried away from the orifice by the extrudate and forms lumps in thewire coating. Lumps are measured optically by laser measurement ofchanges in the diameter of the insulation. An increase in diameter of atleast 50% is considered a lump. The test for lumps is conducted in-lineon the insulated conductor. The average of three wire-coating runs(3×13.7 km lengths) is used for each lump determination. The extrusionwire coating is carried out by conducting a a series of extrusion/meltdraw-down experiments using an extruder for melt draw-down extrusioncoating of a copper conductor, all as described in Example 10 of U.S.Pat. No. 5,703,185. 45,000 ft (13.7 km) lengths of fluoropolymerinsulated copper conductor are produced, which are then tested forlumps. In this series of experiments, the following conditions arevaried to cover the range of variation found in the wire coatingindustry, including conditions that produced poor wire coating: drawdownratio, line speed, cone length, melt temperature and color concentrate.The limits for the variables are set sufficiently wide to insure thatthe process experiences high levels of lumps at extreme conditions withthe best commercially available TFE/HFP copolymer. The industry goal isto have no more than 2 lumps/13.7 km length of insulated wire. Theaverage number of lumps for all these extrusion wire coatings is0.4/13.7 km. This gives much better performance than the same series ofextrusion wire coatings carried out using the best commerciallyavailable TFE/HFP copolymer, which gives the average of 4.5 lumps/13.7km.

What is claimed is:
 1. Process for carrying out a chemical reaction withfluoropolymer or with a contaminant in a fluoropolymer, comprising (a)melting said fluoropolymer, (b) contacting said molten fluoropolymerwith reactant in isolation from said melting, said contacting beingcarried out in a reaction zone having free volume, (c) subdividingmolten fluoropolymer in said reaction zone to enable said reactant toeffectively contact said molten fluoropolymer so as to carry out thechemical reaction between said reactant and said molten fluoropolymer,(d) devolatilizing the resultant molten fluoropolymer in isolation from(b) and (c), and (e) cooling the devolatilized fluoropolymer.
 2. Theprocess of claim 1 and additionally sparging said devolatilizedfluoropolymer.
 3. The process of claim 1 wherein said moltenfluoropolymer of step (a) contains acid end groups or -carboxylate saltend groups and said reactant contains a proton source, said reactionbeing between said proton source and said end groups to form stablehydride end groups (—CF₂H) on said molten fluoropolymer in step (c). 4.The process of claim 3 wherein said reactant is water.
 5. The process ofclaim 3 where said reaction zone is free of added oxygen.
 6. The processof claim 1 wherein said molten fluoropolymer of step (a) containsnon-perfluorinated end groups and said reactant contains fluorine, saidreaction being between said fluorine and said end groups to form stablefluoromethyl groups (—CF₃).
 7. The process of claim 1 wherein saidmolten fluoropolymer of step (a) contains acid end groups and saidreactant contains an amine to form amide end groups.
 8. The process ofclaim 1 wherein said molten fluoropolymer of step (a) contains acid endgroups and said reactant contains ammonia to form —CONH₂ end groups. 9.The process of claim 1 wherein said molten fluoropolymer of step (a)contains acid end groups and said reactant contains an alcohol, to formester end groups.
 10. The process of claim 1 wherein said moltenfluoropolymer of step (a) contains contaminant and said reactantconverts said contaminant to a volatile form.
 11. The process of claim 1wherein the said reactant contains fluorine.
 12. The process of claim 1wherein the said reactant is elemental fluorine.
 13. The process ofclaim 1 wherein the said reactant contains oxygen.
 14. The process ofclaim 1 wherein said subdividing includes countercurrent flow of streamsof said molten fluoropolymer in said reaction zone.
 15. The process ofclaim 1 wherein said subdividing includes dividing said moltenfluoropolymer in said reaction zone into at least 6 streams of moltenfluoropolymer a plurality of times within said reaction zone.
 16. Theprocess of claim 1 wherein said subdividing is equivalent to saidfluoropolymer containing —COOH end groups and said reactant being waterto obtain stable —CF₂H end groups without degrading said fluoropolymer.17. The process of claim 16 wherein the resultant fluoropolymer has awhiteness index of at least about
 60. 18. The process of claim 16wherein said fluoropolymer is tetrafluoroethylene/hexafluoropropylenecopolymer and sparging said copolymer to obtain said copolymer having abackbone volatiles index of no greater than about
 30. 19. The process ofclaim 1 when said subdividing is characteristic of dispersive mixing.